1. Field of the Invention
The present invention relates to a diffuse reflection output conversion method used in detecting the amount of attached powder, such as toner, an attached powder amount conversion method using the diffuse reflection output conversion method, and an image forming apparatus such as a copier, a printer, a facsimile, a plotter, and a multi-function peripheral (MFP) having at least one of these functions, including a function executing these methods.
2. Description of the Related Art
To achieve a stable image density in a conventional electrophotographic image forming apparatus, such as a copier and a laser beam printer, toner patches (gradation patterns) for detecting density are formed on an image carrier such as a photosensitive element. The density of the patches are detected using an optical detector (hereinafter, also simply referred to as a sensor), and the developing potential is adjusted (specifically, the power of a laser diode (LD), a charging bias, and a developing bias are adjusted) based on the detection result.
As a means for detecting the patches for density detection, reflective sensors are generally known. In a reflective sensor, a light emitting diode (LED) as a light emitter (light emitting means) is combined with a photodiode (PD) or a phototransistor (PTr) as a light receiver (light receiving means).
As types of these sensor, a sensor detecting only a specular reflection illustrated in FIG. 2 (see Japanese Patent Application Laid-open No. 2001-324840, for example), a sensor detecting only a diffuse reflection illustrated in FIG. 3 (see Japanese Patent Application Laid-open No. H5-249787 and Japanese Patent No. 3155555, for example), and a sensor detecting the both illustrated in FIG. 4 (see Japanese Patent Application Laid-open No. 2001-194843, for example) are available.
In FIGS. 2, 3, and 4, the reference numerals 50A, 50B, and 50C represent an element holder. The reference numeral 51 represents an LED, and the reference numeral 52 represents a specular reflection receiver. The reference numeral 53 represents a surface to be detected, the reference numeral 54 represents a toner patch formed on the surface to be detected, and the reference numeral 55 represents a diffuse reflection receiver.
In a sensor having one light emitter and two light receivers illustrated in FIG. 4, characteristics of a diffuse reflection output vary greatly, e.g., due to fluctuations in light emitters and light receivers from different lots, the temperature characteristics and aging of the light emitter and the light receivers, and aging of the transfer belt that is the surface to be detected, as described in Japanese Patent No. 4456828. Therefore, the diffuse reflection receivers need to be calibrated in the manner described below.
A known method for calibrating the diffuse reflection output of a sensor includes drawing gradation patterns, detecting the patterns using a specular reflection receiver that has already been calibrated, obtaining a diffuse reflection output at a reference belt surface exposure ratio from a calculated reference belt surface exposure ratio, which is in one-to-one correspondence with the amount of toner attached, and comparing the actual diffuse reflection output with the reference diffuse reflection output at the reference belt surface exposure ratio. The validity of this calibration method has already been confirmed.
Because a specular reflection output reaches the maximum level on the belt surface, it is known that calibration is performed by adjusting the maximum specular reflection output to a reference specular reflection output.
When area coverage modulation patterns are used for density detection, dot unevenness in a pile height direction and area directions might be present in the half-tone portion.
When analog patterns are used, there might be density unevenness in the patterns as illustrated in FIG. 9, because of a beam pitch variation that could occur in a high-speed unit performing two-beam writing, or banding formed in development.
If such unevenness is present, the belt surface becomes more visible locally, even when the amount attached toner is the same. As a result, the specular reflection output might be increased, and the belt surface exposure ratio might be calculated incorrectly, as illustrated in FIG. 10.
Therefore, the diffuse reflection output cannot be calibrated accurately.
There is need of accurate calibration to be performed in a higher end of the amount of attached powder where dot unevenness or density unevenness is less prominent.